A technology of a collecting part structure of an exhaust manifold is disclosed in Japanese Patent applications Publication No. 2004-245156 and No. 2004-116382, and they include a plurality of branch pipes, a collecting part, a partition plate and a sensor attachment boss part, where the branch pipes have exhaust-gas upstream side end portions that are connected with a head flange of an engine, the collecting part collects and contains exhaust-gas downstream side end portions of the branch pipes, the partition plate has an exhaust-gas upstream side end portion that is fixed among the exhaust-gas downstream side end portions of the branch pipes and an exhaust-gas downstream side end portion that is arranged to project in an interior of the collecting part, and the sensor attachment boss part is welded on an outer circumferential portion of the collecting part to face an attachment hole formed in the collecting part.
In addition, the partition plate is provided to obtain objects for a function for portioning the branch pipes when the branch pipes are welded with each other, an improvement in sensitivity of an oxygen sensor and a prevention of interference of exhaust gasses discharged through the branch pipes. A portion of the partition plate is fixed by welding on a portion of the outer circumferential portion of the collecting part in a state where the partition plate is positioned by the part thereof being inserted into a slit formed in a portion of the collecting part along from an exhaust-gas upstream side end portion to an intermediate portion of the collecting part.